Drive system for acoustic devices

ABSTRACT

A drive system for acoustic devices comprising drive devices, the projecting driving studs (14,15) of which are connected to pressure beams (27,28) of the acoustic device, the drive devices comprising a fixture frame (26) inside of which there are located drive units with an intermediate mechanical prestress device (22,23,24,25), the drive units comprising stator and drive cells stacked by means of guide rings (4,5) and guide discs (11), the stator cell comprising a magnetizing coil (1) with a surrounding tube (2) of soft-magnetic material fixed inside a fixture tube (3), and the drive cell comprising a cylindrical magnetic pellet (6) , discs of soft-magnetic material (7,8) and of permanent-magnetic material (9,10) making contact with said magnetic pellet.

TECHNICAL FIELD

The invention relates to a mechanical drive system for use, inter alia,in acoustic devices. Current acoustic devices are able to operate astransmitters, i.e. as transducers for acoustic signals, and as receiversof acoustic signals. An acoustic device in which the invention may beused to great advantage is as a so-called Sonar, that is, a transmitterwhich sends out sound waves under water which, after reflection, can bemonitored by hydrophones of various kinds.

BACKGROUND ART, THE PROBLEM

It is a well-known fact that low-frequency sound waves can travel longerdistances through water than can high-frequency sound waves. For a longtime there has also been a considerable need of powerful low-frequencysound transmitters which are capable of working under water, both from amilitary point of view and from the point of view of the offshore oiland gas industry. Transmitters of various designs and embodiments forthese purposes and fields of use have been available on the market forquite a long time. A summary of such acoustic transmitters is given inan article in DEFENSE SYSTEM REVIEW, November 1984, pages 50-55,entitled "Sonar transducer design incorporates rare earth alloy".

Most acoustic transmitters which are used at present are based on eitherthe piezoelectric effect or on magnetostriction. As is well-known, thepiezoelectric effect means that a crystalline substance is subjected toa change in shape when an electric voltage is applied to its endsurfaces and that a voltage is obtained when the substance is subjectedto a physical deformation, respectively. Magnetostriction means that amagnetic material which is subjected to a change of the magnetic fluxsuffers a change in shape and that an externally caused change in lengthgives rise to a change in the magnetic flux, respectively. This meansthat a transmitter which utilizes these effects can also, in principle,be used as a receiver.

A variety of different embodiments of acoustic transmitters exist. Inlow-frequency applications it is common that they have a cylindricalshape with either a circular or elliptical cross section area.

The greatest problem with this type of transmitters is to achieve asufficiently great amplitude of the oscillations. To this end, either alarge transmitter area or a small transmitter area with great amplitudeof oscillation would be required.

The introduction of the so-called giant magnetostrictive materials hasimproved the conditions for obtaining good acoustic transmitters. Withsuch materials as driving elements, amplitude changes may be obtainedwhich largely amount to 30 times the corresponding changes usingpiezoelectric materials. Transmitters which utilize these giantmagnetostrictive materials have existed for several years. One propertyof transmitters which utilize giant magnetostrictive materials is thatthey must be mechanically prestressed. This can be done in differentways, for example as shown in U.S. Pat. No. 4,438,509 with the aid ofprestressed wires.

A frequently occurring embodiment for the actual driving will bedescribed in greater detail starting from a cylindrical transmitter witha near elliptical cross section. The cylindrical envelope surfaceconsists of an elastic diaphragm or shell. Inside and parallel to theaxis of the cylinder and making contact with the shell are two beamsapplying pressure to the shell. The cross sectional area of the beams issymmetrically mirror-inverted in relation to the minor axis of theelliptical shell and each beam is delimited by that part of the shellwhich faces the end of the major axis and a chord parallel to the minoraxis. Between the beams and making contact with their plane-parallelsides there is arranged an electrically-controlled driving element inthe form of a driving rod. The longitudinal axis of the driving rodcoincides with the major axis of the elliptically-formed cross sectionand lies midway between the end surfaces of the transmitter. In thosecase where the magnetostrictive effect is utilized, the driving rodconsists of a magnetostrictive material which with a surrounding windingis magnetized to keep pace with the desired frequency of thetransmitter. If the piezoelectric effect is to be utilized, the drivingrod consists of a piezoelectric material. The driving rod may, ofcourse, consist in its entirety, or in certain parts, of a material withthe desired possibilities of changing the length.

The fundamental embodiment of an acoustic transmitter described abovemay be different as regards the actual details. An acoustic transmitterwith a cylindrical shape and with an elliptical cross section area andwith driving rods of a giant magnetostrictive material is disclosed,inter alia, in U.S. Pat. No. 4,901,293 entitled "A rare earthflextensional transducer".

Swedish patent 8901905-3, "Device in acoustic transmitters", alsodescribes a cylindrical transmitter with elliptical cross section. Thedriving element here consists of a body with oppositely located recessesinto which driving rods are inserted. The driving rods, in turn, arefixed into pressure rods which in the same way as above influence thediaphragm.

Swedish patent application 9003086-7 describes a drive package foracoustic transmitters comprising a frame of magnetic material withwindows for mounting driving members and prestress devices. Two windowswith driving members and an intermediate window with a mechanicalprestress device form a column which, by means of pressure studs in thedriving members and holes in the frame, prestress pressure beams,located inside the transmitter, in the shell of the transmitter. Thedrive package may comprise several columns.

The building system embraced by the invention comprises magneticcircuits for magnetization of the active material in accordance withU.S. Pat. No. 4,914,412, "Magnetic circuit". The magnetic circuitsincluded are intended to magnetize cylindrically shaped pellets ofmagnetostrictive material, in the axial direction in accordance with theU.S. patent. This magnetic circuit comprises a magnetizing coil,disc-shaped permanent magnets for bias magnetization and discs ofsoft-magnetic material which have a diameter corresponding to theoutside diameter of the coil as well as a soft-magnetic cylindrical tubewhich surrounds the magnetizing coil. The soft-magnetic parts areincluded in the magnetic circuits which comprise the magnetostrictivepellets.

SUMMARY OF THE INVENTION

The drive system for an acoustic device according to the inventioncomprises a number of drive devices placed in parallel and symmetricallybetween the pressure beams which, according to the state of the art, areincluded in these devices. The drive devices comprise a fixture frameinside of which there are two drive units with an intermediatemechanical prestress device. The drive devices have axially extendingstuds which make contact with the opposite pressure beams included inthe acoustic device.

The drive units, in their turn, comprise an optional number of drivingelements stacked in a row. Each driving element consists of a statorcell, drive cell and guide device in the form of guide rings and ahollow guide disc, which together, among other things, realize magneticcircuits in accordance with U.S. Pat. No. 4,914,412.

Each one of the above-mentioned parts included in the drive system willbe given a short summary description. A stator cell consists of theabove-mentioned magnetizing coil which is fixed to a surroundingsoft-magnetic cylindrical tube according to the above, the tube, inturn, being fixed to a similarly cylindrical tube, hereafter called afixture tube, of non-magnetic material. As will become clear from thedetailed description of the preferred embodiments, the soft-magnetictube has an axial length which is somewhat greater than the axial lengthof the coil and the fixture tube a still somewhat greater axial length.

A drive cell according to the invention comprises the above-mentionedcylindrically-shaped magnetostrictive pellet as well as theabove-mentioned soft-magnetic discs concentrically connected to the twocircular end surfaces of the pellet and, making contact with each one ofthese, the discs of permanent-magnetic material. All of these discs havea outside diameter corresponding to the outside diameter of the coil.

For centering and radial guiding of the discs inside the stator cellsand for stacking the fixture tubes, these tubes are turned out for theguide rings at the annular end surfaces. For centering and radialguiding of the magnetostrictive pellet, there has been applied, on oneside of one of the soft-magnetic discs facing the pellet, a thin holedguide disc with an outside diameter equal to that of the discs and withan inside diameter insignificantly larger than the diameter of thepellet.

When stacking the driving elements into a drive unit, the stator cellswill have a common axial centre line via the guide rings. In this way,the drive cells will also be stacked and form a drive cell package witha common pemanent-magnetic disc and a guide ring between each drivecell. The play between the pellets and the holed guide discs is so largethat the movable stack becomes completely parallel to the stator cellstack in spite of the fact that there is no complete parallelism betweenthe contact surfaces included in the movable stack. The function of theguide rings used is, inter alia, to achieve parallelism between thestator cell stack and the movable stack during the mounting and toensure that these stacks become freely movable in the axial direction.

By stacking a suitable number of driving elements on top of each other,drive units with different lengths may be composed.

In order for the drive unit to function in the intended way, it must bemechanically prestressed, as is also clear from the above description ofthe background art. According to the invention, this is achieved bymounting two drive units with an intermediate mechanical prestressdevice inside a fixture frame. The frame with drive units and prestressdevice forms a drive device, which per se constitutes a building elementwhich, in addition to being used in acoustic devices, can also begenerally used as a force- and movement-imparting device for otherapplications.

As described above, the drive system comprises a number of paralleldrive devices, the axial length and number of which are determined bythe dimensions of the surrounding casing and by the force and movementwhich are required in each particular case. By providing the fixtureframes of the drive devices with devices for fixing the frames to eachother, the drive devices can be mounted together to form a completedrive system consisting of the desired number of drive devices. Fixtureplates can also be mounted on the drive devices, and on these fixtureplates any auxiliary equipment may be placed.

Acoustic devices according to the invention have an efficiency outsidethe resonance frequency range which is normally lower than 50%. Thismeans that, in continuous operation, the parts included have to becooled. Cooling of the drive system according to the invention maysuitably be performed by providing the external rings of the statorcells with cooling channels or cooling flanges. In the patentapplication entitled "Cooling system for acoustic devices", filedconcurrently with this application, a cooling system with coolingchannels in the stator cells is described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a stator cell with guide rings.

FIG. 2 shows a drive cell with a holed guide disc.

FIG. 3 shows the composition of a drive unit.

FIG. 4 shows how a drive device is built up.

FIGS. 5 and 6 show the composition of a drive system and how it may bebuilt into an acoustic device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a stator cell with guide rings. The stator cell comprises amagnetizing coil 1 fixed to a soft-magnetic tube 2 which in turn isfixed to a fixture tube 3. In the end surfaces of the fixture tubes,slots for the guide rings 4 and 5 have been made. As is clear from thefigure, the guide rings are of somewhat different design and theexplanation of this will be given below.

FIG. 2 shows the composition of a drive cell with an associated guidedisc. It comprises a cylindrically-shaped magnetic pellet 6 with discs 7and 8 of soft-magnetic material making contact with the two end surfacesof the pellet. Permanent-magnetic discs 9 and 10 make direct contactwith the discs 7 and 8. The diameters of the discs largely correspond tothe outside diameter of the magnetizing coil. For radially guiding thepellet, a thin holed guide disc 11 of a suitable plastic material isprovided on that side of the soft-magnetic disc 8 which faces thepellet, the guide disc having the same outside diameter as the discs andhaving an inside diameter insignificantly greater than the diameter ofthe pellet.

FIG. 3 shows a drive unit composed of an optional number of stackedstator cells and a corresponding number of drive cells. As the figureshows, the stacked drive cells form a drive cell package with a commonpermanent-magnetic disc between each drive cell. Towards the externalstator cells of the drive unit, edge rings 12 and 13 are connected. Theoutwardly-facing surface of these rings may be in the form of a frustumof a cone. As is shown, the guide rings 5a and 5b for centering the edgerings are shaped somewhat differently from the guide rings 4 between thestator cells since the guide rings 5a and 5b only need to center onesoft-magnetic and one permanent-magnetic disc. A suitable material forthe guide rings is copper which provides a certain lubricating effect inoperation while at the same time its good thermal conductivity makespossible a good transport of heat out to the fixture tubes. The outputof force and movement of the drive unit is performed at the two drivingrods 14 and 15 which, towards the permanent-magnetic discs of theexternal drive cells, are shaped as discs 16 and 17.

A driving element according to FIG. 3 can be mechanically prestressed ina plurality of different ways, for example as stated in theabove-mentioned U.S. Pat. No. 4,438,509. However, a preferred embodimentof the invention is shown in FIG. 4 which constitutes a so-called drivedevice. The drive device comprises two drive units which are practicallyidentical with the drive unit in FIG. 3 and with an intermediatewedge-shaped mechanical prestress device. The difference between thedrive units is that the edge rings which are facing the prestress devicehave been replaced by the parallelepipeds 18 and 19. The drive studs 20and 21 facing the prestress device may also be somewhat adjusted in theaxial length to adapt to the dimensions of the parallelepipeds.

Recesses for two lugs 22 and 23 with confronting plane surfaces, makingan angle equal to the wedge angle of a prestress wedge 24 placed betweenthe lugs, have been provided in the parallelepipeds. A hole for aprestress screw 25 is threaded in the wedge. The drive studs 20 and 21make contact with the intermediate prestress device, which is fixed to afixture frame 26. The drive studs 14 and 15 extending from the drivedevice make contact with the two pressure beams 27 and 28 of theacoustic device.

The fixture frame is of ferromagnetic material and constitutes anintegral part of the magnetic circuit which closes the magnetic fluxemanating from the discs of permanent-magnetic material located at thetwo outer ends of the drive device.

In two views perpendicular to each other, FIGS. 5 and 6 show how a drivesystem according to the invention may be built up in a preferredembodiment and how, shown in dashed lines, it may be built into anacoustic device of cylindrical shape with a near elliptical crosssection. The shell of the acoustic device is shown at 29. In thisembodiment the drive system comprises four drive devices which aredirectly screwed together two-by-two via extending arms 30 and 31 on thefixture frames. These directly screwed-together drive devices are alsoscrewed together by means of spacing yokes in such a way as to form anintermediate space for electric control means, etc. These control meansare suitably mounted on a fixture plate as shown at 32. As is clear, theprestress mechanism in the drive devices according to FIGS. 3 and 4 hasbeen rotated through 90 degrees in relation to the prestress mechanismin FIG. 5. The counter support for the prestress screw 25 now consistsof an element 33 fixed between the parallelepipeds 18 and 19. In apreferred embodiment, the heads of the prestress screws are shaped asworm screws, which can all be operated by worm screw rods extendingtowards one of the end plates.

The outer drive devices are fixed by screws 34, 35, 36 and 37 to therespective end plates 38 and 39 of the acoustic device. The movabilityand sealing of the shell are ensured by means of the elastic rings 40and 41 provided around the end plates.

A drive system according to the invention is not limited only to thepreferred embodiment shown in the accompanying figures. Thus, forexample, the drive unit may consist of anything from one to severaldriving elements. Similarly, the drive device may also comprise anythingfrom one to several drive units. In a drive device comprising only onedrive unit, one of the parts of the prestress mechanism will makecontact with the fixture frame and one of the driving rods will extendfrom the prestress mechanism. The drive system may, of course, alsocomprise only one drive device.

We claim:
 1. A drive system for an acoustic device, said acoustic drivedevice having a cylindrical elastic shell and diametrically opposedpressure beams positioned within and in contact with said shell, saiddrive system being positionable between said opposed pressure beams andcapable of moving the shell in an oscillating motion, said drive systemcomprising a drive device which includes:an elongated hollow framedefining opposite ends with openings therein, first and second driverods respectively positioned within said frame at said opposite endsthereof and providing portions which extend through the openings in therespective ends of said frame so as to contact and oscillate saidpressure beams, a mechanical prestress key joint means positioned withinsaid frame between said first and second drive rods, and a driveassembly operating as a magnetic circuit positioned within said framebetween said first and second drive rods in stacked relation to saidmechanical prestress means, said drive assembly comprising:a stator cellincluding a magnetizing coil, a first tube of soft magnetic materialsurrounding said coil, a second tube of non-magnetic materialsurrounding said first tube, and guide rings at opposite ends of saidstator cell, and a drive cell including a cylindrical magnetic pelletwhich extends through said magnetizing coil of said stator cell, firstand second discs of soft magnetic material respectively in contact withopposite ends of said pellet, third and fourth discs of permanentmagnetic material respectively in contact with said first and seconddiscs, and a disc guide for radially guiding the pellet, oscillatingcurrent supplied to said magnetizing coil causing an oscillating motionof said drive rods and thus said pressure beams and said acousticdevice.
 2. A drive system for an acoustic device according to claim 1,comprising four said drive devices and wherein each drive devicecomprises two pair of stacked drive assemblies separated by a saidmechanical prestress key joint means.
 3. A drive system for an acousticdevice according to claim 1, wherein said drive device comprises atleast four stacked stator cells and drive cells.
 4. A drive system foran acoustic device according to claim 1, wherein said key joint meanscomprises a wedge defining two plane surfaces extending at a wedge anglerelative to one another, two lugs with plane surfaces which contact therespective plane surfaces of said wedge, and a screw for pulling thewedge relative to the two lugs such that the two lugs will move apart tocreate mechanical prestress in the magnetic pellet.